In video tape recorders (hereinafter referred to as VTR), such as "Hi-Fi VTRs", a so-called deep layer recording technique has been recently developed. In this technique, two channel audio signals are modulated with different frequencies and then multiplexed with each other. The multiplexed FM (frequency modulation) audio signal is recorded on an audio track of a video tape over a relatively thick layer thereof.
In a reproducing operation, the multiplexed FM audio signal is reproduced from the audio track of the video tape by a reproducing head. Then two channel audio signals are separated from the multiplexed FM audio signal, by using band pass filters (BPFs) with different frequency bands.
Shown in FIG. 1 is a block diagram illustrating an audio system of conventional Hi-Fi VTRs. Two channel audio signals, i.e., left (L) channel and right (R) channel audio signals, are applied to L channel and R channel de-emphasis encoders 2, 3 of a recording system. These encoders 2, 3 constitute a well-known noise reduction system in cooperation with emphasis decoders 15, 16 in a reproduction system, which will be referred later. In the encoders 2, 3, the L channel and R channel audio signals have their amplitudes expanded at a prescribed frequency level to remove noise components. The expanded amplitude audio signals are applied to FM modulators 4, 5, respectively. The L channel, R channel FM signals are then combined together in a mixer 6. The combined signal is supplied to a recording head 8 after amplified by an amplifier 7. The signal is then recorded on an audio track of video tapes (not shown).
In a reproducing system, the recorded signal is reproduced through a reproducing head 9. The reproduced signal has the configuration of the multiplexed FM audio signal. The reproduced signal from the reproducing head 9 is then applied to both an L channel BPF 11 and an R channel BPF 12 through an amplifier 10. These BPFs 11, 12 have different frequency bands which correspond to the frequencies of the FM modulators 4, 5, respectively. Thus, the L channel FM audio signal is extracted from the L channel BPF 11, while the R channel FM audio signal is extracted from the R channel BPF 12. These FM audio signals are applied to FM demodulators 13, 14 so that audible band L channel and R channel audio signals are demodulated therefrom. These audible band L channel and R channel audio signals are applied to the above-mentioned emphasis decoders 15, 16. In the emphasis decoders 15, 16, the L channel and R channel audio signals have their amplitudes compressed at a prescribed level which is complementary to those of the de-emphasis encoders 2, 3. Thus the original L channel and R channel audio signals are obtained through output terminals.
In these Hi-Fi VTRs, such audio recording and reproducing systems have been increasingly integrated into one chip ICs. However, the BPFs 11, 12 are difficult to incorporate into such one chip ICs, because a remarkable fluctuation of frequency characteristics thereof. When an ordinary semiconductor manufacturing process is used, resistance and capacitance, which determine the frequency characteristics of filters, generally fluctuate by about .+-.20%.
The center frequencies of the L channel and R channel FM audio signals of VHS VTR have been specified at 1.3 MHz and 1.7 MHz with maximum deviation .+-.50 KHz. Therefore, the passbands of the BPFs described above have been set as follows: EQU L channel: 1.15 to 1.45 MHz EQU R channel: 1.55 to 1.85 MHz
That is, these passbands are isolated by a narrow band of only 1,000 KHz so that the channel separation performance can be remarkably deteriorated unless the centers of the BPFs are precisely realized.
If the BPFs 11, 12 are manufactured using the semiconductor manufacturing process described above, fluctuation of the center frequencies (with resistance and capacitance fluctuation .+-.20%) become too large as expressed as follows: EQU L channel: 0.83 to 1.83 MHz EQU R channel: 1.09 to 2.45 MHz
This shows that their fluctuation ranges overlap each other so that it is difficult to incorporate the BPFs into ICs.
So as one technique to incorporate filters into ICs, a combination of filters and adjusting circuits for the filters is conventionally employed.
FIG. 2 shows an R channel filter 27 in the form of a BPF and an adjusting circuit for the BPF 27. In FIG. 2, an FM audio signal is applied from an input terminal 27a to the BPF 27. An output of the BPF 27 is led to an output terminal 27b. The time constant of the BPF 27 is adjusted by an automatic time constant adjusting circuit 18 as follows. The automatic time constant adjusting circuit 18 is constituted by an integration circuit 17, a switch 21, a peak hold circuit 20, a comparator 19 and a reference voltage source 25. The integration circuit 17 includes a variable transconductor 24 with its input terminals supplied with a reference voltage Vref from a reference voltage source 25, a capacitor 22 coupled to the output terminal of the transconductor 24 and a variable current source 23 for biasing the transconductor 24. The switch 21 is connected in parallel to the capacitor 22 for periodically short-circuiting the capacitor 22 under the control of a clock signal. Thus a triangular pulse signal as shown in the drawing is generated on the terminal of the capacitor 22.
The triangular pulse signal is applied to the peak hold circuit 20, wherein the peak voltage of the triangular pulse signal is held. The peak voltage is supplied to an input terminal of the comparator 19. The comparator 19 compares the peak voltage with the reference voltage Vref. Then a difference voltage obtained on the output terminal of the comparator Ig is supplied to a capacitor 26 having a large capacitance for charging it. The terminal voltage of the capacitor 26 is applied to both the BPF 27 and the variable current source 23. The voltage applied to the current source 23 causes the control current of the current source 23 to vary. Thus, the integration circuit 17, the switch 21, the comparator 19, the capacitor 26 and the current source 23 constitute a control loop for automatically adjusting the time constant of the transconductor 24.
According to the control loop, the transconductance gm of the transconductor 24 is varied by a difference between the peak level of the triangular pulse signal and the reference voltage Vref. The variation of the transconductance gm causes the amount of a current applied to the capacitor 23 from the transconductor 24 to vary. Thus, the slope of the triangular pulse signal varies in response to the amount of the current. Thus, when the peak level is higher than the reference voltage Vref, the slope becomes lower. On the other hand, when the peak level is lower than the reference voltage Vref, the slope becomes higher. Thus, the peak level of the triangular pulse signal comes to agree with the reference voltage Vref. Under the controlled status the following relation is maintained. EQU T//2=Co/gm (1)
wherein T denotes a pulse period of the clock signal and Co denotes the capacitance of the capacitor 22.
The above equation (1) represents that the right side, which represents the time constant of the integration circuit 17, is defined by only the pulse period T. In other words, the time constant of the integration circuit 17 is precisely adjusted if the clock signal has a stable frequency.
Here, it is assumed that the automatically adjusted BPF 27 also consists of a variable transconductor of the same type as the transconductor 24 in the automatic time constant adjusting circuit 18. It is also assumed that an adjusting signal Va applied to the BPF 27 from the automatic time constant adjusting circuit 18 is the same as the signal applied to the current source 23 of the integration circuit 17. Under these conditions, the transfer function H(S) of the BPF 27 subjected to the adjustment, which is the same as that of the transconductor 24, is expressed by the following equation. EQU H(S)=F([Co/gm].multidot.S) (2)
Therefore, the time constant of the BPF 27 is accurately decided by the automatic time constant adjusting circuit 18. Thus the frequency characteristics of the BPF 27 is decided relying only the accuracy of the automatic time constant adjusting circuit 18, i.e., the error of the time constant Co/gm of the integration circuit 17.
However, it is well known that a high accuracy of the time constant Co/gm is difficult to achieve in the conventional system as shown in FIG. 2, due to several error factors as described below.
a) In the automatic time constant adjusting circuit 18 itself, due to an adjusting error caused by an offset of the reference voltage Vref, etc.
b) Mismatch between the variable transconductor 24 and the capacitor 22 of the automatic time constant adjusting circuit 18 and the variable transconductor and the capacitor in the BPF 27 to be adjusted.
c) Mismatch between the variable current source 23 of the automatic time constant adjusting circuit 18 and a corresponding variable current source in the BPF 27.
In particular, in large scale integrated circuits (LSIs) a plurality of filters are often adjusted collectivley by one automatic adjusting circuit. In this case, it is difficult to incorporate the automatic adjusting circuit and filters on an IC chip adjoining or located closely to each other and characteristic errors due to the error factors as described in a) and b), tend to occur. It is therefore the present state that adjusting accuracy still has an error of about .+-.5%. This amount of error has been considerably improved when compared with the error of about .+-.40% in the case of unadjusted filters, but it is still insufficient.
Now, by applying the above concepts to the channel separating BPFs of the "VHS Hi-Fi" VTR described above, assuming that the control characteristic of those filters are controlled by the automatic adjusting circuit having a .+-.5% error, the center fluctuations of the filters are as follows: EQU L channel: 1.235 to 1.366 MHz EQU R channel: 1.615 to 1.785 MHz
For instance, such as BPF has a sharp attenuation characteristic as shown in FIG. 3. If the passing band center frequency of such a filter deviates largely from the actual FM signal frequency, not only the channel separation is deteriorated, but, when FM signals having wide spectra in case of large amplitude or when high modulation frequencies are applied, distortion after demodulation is also increased.
It is therefore indispensable for BPFs to have a means to perform the manual adjustment from the outside and furthermore, the adjustment is necessary for every channel.
As described above, the conventional automatic filter adjusting system requires an accurate clock and a capacitor having a large capacitance but it has such defects that temperature drift and voltage drop drift tend to occur and their compensation is difficult and furthermore, circuit scale can become large.